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ABSTRACT The use of tools to drink water is well‐documented in wild chimpanzees, but the specific function of this behavior is unclear. Here we use a large data set of drinking behaviors spanning 14 years of observation from the Kanyawara community of chimpanzees living in Kibale National Park, Uganda, to test two possible functions of leaf‐sponges and other drinking tools. On the one hand, chimpanzees may use tools to access water that is hard to reach, which predicts that chimpanzees will preferentially use tools to drink at tree holes and crevices compared to all other locations. Conversely, chimpanzees may use these tools to filter stagnant water, in which case they would use tools more often at holes and puddles compared to running water sources (e.g., streams). We compared both likelihood of using a tool to drink at different locations, as well as overall rates of drinking, and found chimpanzees in this community most often drink from streams without tools. However, when they do use tools, they preferentially do so to drink at tree holes. Given known age and sex effects on tool use in chimpanzees, we also examined demographic variation in drinking tool use to understand the emergence of this behavior. While females use tools more often than males overall—in part driven by differences in drinking rates at different locations—both males and females use tools more frequently at tree holes than other locations when they do drink there. Finally, comparisons by age indicate that this selectivity strengthens over development with older chimpanzees showing a more pronounced effect of using tools more often at tree holes, suggesting that younger chimpanzees may exhibit exploratory tool use behavior. These results pinpoint the specific function of tool use during drinking and further suggest that even simple tools may require learning for use in appropriate contexts. 

Sex differences in physical aggression occur across human cultures and are thought to be influenced by active sex role reinforcement. However, sex differences in aggression also exist in our close evolutionary relatives, chimpanzees, who do not engage in active teaching, but do exhibit long juvenile periods and complex social systems that allow differential experience to shape behavior. Here we ask whether early life exposure to aggression is sexually dimorphic in wild chimpanzees and, if so, whether other aspects of early sociality contribute to this difference. Using 13 y of all-occurrence aggression data collected from the Kanyawara community of chimpanzees (2005 to 2017), we determined that young male chimpanzees were victims of aggression more often than females by between 4 and 5 (i.e., early in juvenility). Combining long-term aggression data with data from a targeted study of social development (2015 to 2017), we found that two potential risk factors for aggression—time spent near adult males and time spent away from mothers—did not differ between young males and females. Instead, the major risk factor for receiving aggression was the amount of aggression that young chimpanzees displayed, which was higher for males than females throughout the juvenile period. In multivariate models, sex did not mediate this relationship, suggesting that other chimpanzees did not target young males specifically, but instead responded to individual behavior that differed by sex. Thus, social experience differed by sex even in the absence of explicit gender socialization, but experiential differences were shaped by early-emerging sex differences in behavior.